1. Field of the Invention
The embodiments of the invention generally relate to microelectronic systems, and more particularly to microelectromechanical systems (MEMS) and MEMS actuation and sensing technology.
2. Description of the Related Art
MEMS devices are micro-dimensioned machines manufactured by typical integrated circuit (IC) fabrication techniques. The relatively small size of MEMS devices allows for the production of high speed, low power, and high reliability mechanisms. The fabrication techniques also allow for low cost mass production. MEMS devices typically include both electrical and mechanical components, but may also contain optical, chemical, and biomedical elements.
There are a number of actuation and sensing technologies utilized in MEMS; the most common are electrostatic, electrothermal, magnetic, piezoelectric, piezoresistive, and shape memory alloy technologies. Of these, electrostatic MEMS are by far the most common due to their simplicity of fabrication and inherent electromechanical capabilities. However, piezoelectric MEMS out-perform electrostatic MEMS actuators in out-of-plane (vertical) displacements in terms of attainable range, power consumption, and voltage level. Typical electrostatic out-of-plane actuators, parallel plate electrostatic actuators, generally attain vertical displacements on the order of a few microns for several tens of volts while consuming microwatts of power. Piezoelectric out-of-plane unimorph actuators, as depicted in FIGS. 1 through 3, have shown greater than one hundred microns of vertical displacement for five volts and consuming tens of nanowatts of power within an equivalent device area. However, currently, piezoelectric MEMS are limited to out-of-plane motion while electrostatic MEMS excels at in-plane actuation.
The most basic piezoelectric actuator/sensor example is that of the unimorph, a composite piezoelectric cantilever beam. FIGS. 1 through 3 illustrate such a cantilever structure 5 having a supporting beam 10 with a bottom electrode 12, piezoelectric layer 14, and top electrode 16 formed successively over the supporting beam 10, and mechanically fixed at an anchored end 18 opposite a free end 20 of the cantilever structure 5.
In most conventional piezoelectric MEMS actuators, such as the cantilever structure 5 illustrated in FIGS. 1 through 3, out-of-plane bending (plane of bending is the x-z plane) is accomplished by building structures that are asymmetric about the x-y midplane of the piezoelectric layer 14, where the x-axis corresponds to the longitudinal direction, the y-axis corresponds to the width direction, the z-axis corresponds to the thickness (height) direction, and the origin of the coordinate system is located at the center of the clamped end of the cantilever structure 5. The supporting beam 10 acts as an additional structural layer that offsets (by distance δ) the neutral axis (N.A.1) of the supporting beam 10 from the geometric mid-plane of the piezoelectric layer 14 to a neutral axis (N.A.2) located beneath the piezoelectric layer 14. When a voltage is applied between the bottom and top electrodes 12, 16, respectively, a piezoelectrically generated strain induced axial force acts on the cantilever structure 5. The effective line of action of this force lies in the geometric mid-plane neutral axis (N.A.1) of the piezoelectric layer 14. When this axial force acts at some perpendicular distance, δ, (moment arm) from the neutral axis (N.A.1) of the piezoelectric layer 14, a bending moment (M) is created. This bending moment (M) causes the cantilever structure 5 to bend in the x-z plane. The converse effect is true for the structure to function as a sensor. An applied stress, causing bending, will cause the piezoelectric material to generate a voltage which may be detected with additional electronics.
With a top electrode 16 that is symmetric about the x-z-plane, the conventional cantilever structure 5 results in a moment arm (δ) that lies in the x-z plane, as shown in FIG. 3. The induced bending thus occurs in an x-z plane (i.e., out-of-plane). However, such a structure does not facilitate in-plane bending. Clearly, the ability to have a structure capable of both in-plane and out-of-plane motion would be advantageous.
As described in “Recurve Piezoelectric-Strain-Amplifying-Actuator Architecture,”;IEEE/ASME Transactions on Mechatronics, Vol. 3, No. 4, 293-301, December 1998, the complete disclosure of which, in its entirety, is herein incorporated by reference, Ervin and Brei have designed macro-scale (non-MEMS) piezoelectric actuators that utilize what they term “piezoelectric recurve” actuators. These structures generate two distinct equal and opposite piezoelectric bending moments within each basic recurve beam, each affecting its respective half of the beam, that cause the translation of the free end of the actuator. These basic actuators may be connected to provide amplified actuation. However, this conventional design generally cannot be implemented at the MEMS scale without modifications to account for the MEMS specific problems of residual stress deformation and thin film piezoelectric electroding requirements.
Mechanical mechanisms require spatial degrees of freedom for motion. There are six primary degrees of freedom including three translational and three rotational along and about each of the three principle dimensions x, y, and z. The present state of piezoelectric MEMS actuation generally possesses only one of these translational degrees of freedom. Electrostatic MEMS however, the most commonly employed MEMS technology, possesses all three translational degrees of freedom. As a result, there is a much wider array of possible mechanisms and devices that may be constructed with electrostatic MEMS actuators. Electrostatic parallel plate actuators achieve both out-of-plane and in-plane actuation and “combdrive” actuators are commonly used for large in-plane displacements. As such, the lack of piezoelectric actuators with additional spatial degrees of freedom has limited the scope of possible piezoelectric MEMS devices to date. Therefore, there is a need for a novel piezoelectric MEMS actuator/sensor device in which actuated/sensed translation specifically occurs in a lateral (in-plane) direction.